This invention relates to air curtain type refrigerated display cabinets, and more particularly to a novel air defrost refrigerated display cabinet.
Multiple air curtain refrigerated display cases or cabinets have, since the pioneering development of such in the late 1950's and early 1960's by E. W. Simons, S. Beckwith, W. Hagen, R. Vogel, and others, gained wide acceptance in the food market industry. Such cabinets provide tremendous advantages in the storage and display of frozen foods and the like. The cabinets generally employ two or three air curtains, with the innermost one and the adjacent one normally being recirculated around the cabinet through passages. The innermost curtain is normally the coldest, the second one being somewhat warmer, and the third outer one being basically an ambient temperature curtain to reinforce the jet inertia and warm the aisle. Refrigeration means, normally in the form of one or more evaporator coil units, is located in the innermost passage for cooling the air flowing past. Periodically during operation, this innermost passage and its refrigeration means must be defrosted to remove accumulated frost on the coil collected from the cooled air and tending to impede the operation of the equipment. On commercial units, such defrosting is usually achieved with electrical heaters adjacent to the coils of the refrigeration means, or in some instances, by passing hot gas through the coils of the refrigeration means. Hot gas defrost is complex, and is practical in only a small percentage of the installations, however. With electrical defrost, the refrigeration operation is temporarily halted, allowing the recirculating air curtain to be warmed by the high voltage electrical heaters. This normally requires special wiring from the compressor room and, because of the high wattage, is normally operated at higher voltage than the rest of the electrical components of the case. The warm air can then melt the frost built up on the evaporator tubes and fins. It is important to melt this frost, as rapidly as possible in order to minimize temperature rise of the frozen food products, and to minimize collection of frost on the frozen food products from the higher humidity in the recirculated warm air. To achieve this rapid defrosting necessitates the use of considerable electrical power, as is understandable, but has been necessary to this time.
Several years ago, some attempts were made to defrost the display cabinet by circulating ambient air through the inlet and coil zones of the inner passage (U.S. Pat. No. 3,082,612), or the inlet, coil, and back zones of the inner passage (U.S. Pat. No. 3,403,525), using internal baffles to block off the discharge end of the passage. Unfortunately, neither of these concepts proved practical and, as a consequence, all of the thousands of multiple curtain display cabinets manufactured use either the electrical defrost or, in a small percentage of cases, the hot gas defrost technique.
More recently, a cabinet was developed which effectively uses air defrosting, as set forth in U.S. Pat. No. 3,850,003, the defrost air traveling through the coils from both ends. Air defrost has significant advantages over hot gas defrost, as well as significant benefits over electrical defrost. Compared to hot gas defrost, air defrost enables the use of less suction and liquid line, less plexiglass dividers between case sections, less suction and liquid line solenoids, less EPR valves, less 3 way valves, and less auxiliary heat or aisle warming because of the ability to use heat from condensing. Compared to electrical defrost, hot air defrost requires no additional cabinet electrical power for defrosting, the defrost time required is less, and the product temperature rise is less. Unfortunately, however, although the energy savings from the unit in U.S. Pat. No. 3,850,003 are very impressive, and undoubtedly would be important over a long period, the added depth of the resulting cabinet enabled only three twelve foot cabinets to be fitted onto a forty foot long shipping trailer rather than six as previously, thereby causing shipping expense to be double. This added expense plus some added manufacturing expense for the novel defrost construction, enabled competitors to spotlight the differential in initial cabinet cost to the customer, and thereby discourage sale of this cabinet.
Therefore, efforts were applied to achieve an effective air defrost case which could be sold at a price comparable to the energy-consuming competitive units, to thus achieve the energy savings beneficial over the long run to the customer and the nation, while preventing the competition from causing such an energy saving case to be shelved as a result of some initial price differential.